Using VSP to improve interpretation of ocean bottom seismic

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Abstract

Operating companies work continuously on increasing their knowledge about the reservoir.
In mature areas of the North Sea many wells have been drilled, which can be used to obtain
improved images of the subsurface in the area. 3D- and time-lapse (4D) techniques have
evolved making it possible to monitor the production to a large degree. Likewise, seismic
data processing methodologies and processing flows have been designed utilizing the
seismic data to a high degree. However, yet much knowledge is to desire, and traditional
methods only partially succeed. Therefore, additional methods must come in place.
One such method is the use of converted (S) energy in seismic exploration. Anticipating the
same source and similar attenuation mechanism for both P- and S-waves, the resolution of
S-wave images should be superior to that of P-waves. Therefore, complementary
information about the subsurface formations may be obtained by the careful analysis of Swaves.
Due to multi-component seabed acquisition it is now possible to record and process
converted (S) waves also in marine environments. One major drawback however is the poor
quality of the converted energy of such data. It is often difficult to correlate the S-events to
the P-events. This study is an attempt to increase the confidence of correlation between an
S-event and the corresponding P-event on the multi-component data. To accomplish this
task we make use of information from VSP.
VSP (Vertical Seismic Profiling) is a seismic survey method whereby the receivers are
positioned downhole in the wellbore, while the source is located on the surface. VSP has
several advantages. By placing receivers in wells, closer to the structures we aim to map, we
can improve the imaging of formations where the surface seismic fails to achieve
interpretable results. It is also possible to construct seismic image of reflectors which extend
laterally away from the borehole.
In VSP we record both P- and converted (S)-waves. Beneficial to our purpose, the
interpretation of the VSP will give information about mode-conversion taking place in the
subsurface. We are able to tell whether the conversion has taken place as a reflected or a
transmitted conversion (up or down), we may identify the formation tops acting as
converting interfaces, and we may compute the formation interval S-wave velocity with
confidence.
A depth-velocity model incorporating VSP survey geometry, performed over the Snorre field,
was built for both P- and S-waves. The models show the basic geometry of the VSP survey,
and the raypaths, and the S-wave model also shows the mode conversion. The VSP data is
compared to a 3D multi-component seabed seismic survey in the same area. The interpreted
horizons in the 3D volume correlate well with the interpreted horizons of the VSP seismic
profile. It is therefore shown that the use of both P- and S-waves from the VSP data can be
used to correlate the interpretation of the converted (S) 3D volume to the regular P-wave 3D
volume, and in addition provide new constraints on lithology (sand/shale ratio).